Sun28November0333PM 17

The functional residual capacity (FRC):

True / False

Is increased in the obese Correct

Increases on changing from the supine to the standing position Correct

Is approximately 5% higher in men than women Incorrect answer selected

Falls with general anaesthesia Correct

Falls with increasing age Incorrect answer selected

Explanation

The functional residual capacity (FRC) is the volume remaining in the lungs at the end of a passive expiration. It cannot be measured directly and is the sum of the expiratory reserve volume (ERV) and the residual volume (RV). The FRC in a 70 kg man is approximately 3000 mL and in a woman 2500 mL (approximately 17% difference).

General anaesthesia results in the loss of muscle tone and change in respiratory mechanics. There is an alteration in the balance between outward forces (i.e., respiratory muscles) and inward recoil forces (i.e., elastic tissue in the lung) leading to a fall in FRC, typically 400-500 mL in an adult. This is also accompanied by an increase in the elastic behavior of the lung (reduced compliance) and an increase in respiratory resistance. Changing the body position from supine to upright results in an increase of FRC by 800-1000 mL.

As one ages there is a gradual change in lung volumes and capacities. The FRC slightly increases secondary to the loss of elastic recoil of the lung tissue (and increased compliance). Obesity will often negate this and decrease FRC. The most significant change in lung volume with age is the closing capacity (CC) and its relation to FRC. CC approaches the FRC in a 45-year-old in the supine position and exceeds FRC in a 60-year-old in the erect position, leading to small airway closure.

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Average score: 65.75%

Times answered: 233